And McCrum has a helpful explanation for how such miracles may come to pass:

… these books are invariably love stories, in the broadest sense, inspired by a person or a memory – in Twain’s case, of the Mississippi – for whom the writer calls up one final surge of creative energy.

Ah, yes. I think I’ll call up one final surge of energy and totter off downstairs to brew a pot of tea. If you don’t hear further, assume I expired en route to my desk.